Aas 11-509 Artemis Lunar Orbit Insertion and Science Orbit Design Through 2013

AAS 11-509 ARTEMIS LUNAR ORBIT INSERTION AND SCIENCE ORBIT DESIGN THROUGH 2013 Stephen B. Broschart,∗ Theodore H. Sweetser,y Vassilis Angelopoulos,z David C. Folta,x and Mark A. Woodard{ As of late-July 2011, the ARTEMIS mission is transferring two spacecraft from Lissajous orbits around Earth-Moon Lagrange Point #1 into highly-eccentric lunar science orbits. This paper presents the trajectory design for the transfer from Lissajous orbit to lunar orbit insertion, the period reduction maneuvers, and the science orbits through 2013. The design accommodates large perturbations from Earth’s gravity and restrictive spacecraft capabilities to enable opportunities for a range of heliophysics and planetary science measurements. The process used to design the highly-eccentric ARTEMIS science orbits is outlined. The approach may inform the design of future eccentric orbiter missions at planetary moons. INTRODUCTION The Acceleration, Reconnection, Turbulence and Electrodynamics of the Moons Interaction with the Sun (ARTEMIS) mission is currently operating two spacecraft in lunar orbit under funding from the Heliophysics and Planetary Science Divisions with NASA’s Science Missions Directorate. ARTEMIS is an extension to the successful Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission [1] that has relocated two of the THEMIS spacecraft from Earth orbit to the Moon [2, 3, 4]. ARTEMIS plans to conduct a variety of scientific studies at the Moon using the on-board particle and fields instrument package [5, 6]. The final portion of the ARTEMIS transfers involves moving the two ARTEMIS spacecraft, known as P1 and P2, from Lissajous orbits around the Earth-Moon Lagrange Point #1 (EML1) to long-lived, eccentric lunar science orbits with periods of roughly 28 hr. This paper presents the design of this final transition and discusses how the various science objectives were achieved by the transfer and lunar orbit designs. As of this writing, the P1 and P2 spacecraft have both successfully performed their lunar orbit insertion (LOI) maneuvers (on June 27 and July 17, 2011, respectively). Both spacecraft are currently undergoing a series of period reduction maneuvers (PRMs) en route to their final science orbits. Many aspects of the transfer from Lissajous to the science orbits contribute to the novelty of the design. Firstly, the ARTEMIS probes have been the first spacecraft to fly Lissajous orbit near the Earth-Moon La- grange points and thus, they are the first to approach a traditional lunar orbit from this location. Second, the ARTEMIS probes plan to routinely operate in the most eccentric lunar science orbits of any mission to date∗. These orbits are strongly perturbed by the Earth’s gravity, which makes for interesting three-body dynamics ∗Mission Design Engineer, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109. yMission Design Engineer, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109. zDistinguished Visiting Scientist, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109 and Professor, Department of Earth and Space Sciences, University of California - Los Angeles, Los Angeles, CA 90095. xAerospace Engineer, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771. {Aerospace Engineer, NASA Goddard Space Flight Center, Greenbelt, Maryland, 20771. c Copyright 2011. All rights reserved. ∗The SMART-1 spacecraft transitioned a more eccentric orbit upon initial capture at the Moon en route to a less eccentric operational orbit [7]. 1 that are more typically considered in the context of outer planet moon orbiters. Further, ARTEMIS plans a number of different science investigations while in lunar orbit [5, 6], many of which have particular trajectory implications that must be satisfied. Finally, P1 and P2 were built as low-cost Earth orbiters; the spacecraft capabilities, particularly in the area of available thrust and ∆V , are less robust than they may have been if the lunar mission had been planned before launch. The paper begins with a review of the scientific objectives for ARTEMIS and the implications for trajectory design. Next, the lunar orbit dynamics are discussed, along with the qualitative insights and simplifica- tions found to be helpful in the design process. A discussion follows on the design limitations arising from spacecraft and mission constraints. The baseline design solutions for P1 and P2 are then presented, with a discussion of the methodology used to approach key design hurdles. The achieved science opportunities are then shown, confirming that the goals and constraints of the mission are met by the design. LUNAR ORBIT PHASE SCIENCE OBJECTIVES Because ARTEMIS is both a heliophysics and a planetary science mission, a wide range of science obser- vations are planned for during the lunar orbit phase [6], including: • 3-D mapping of the lunar wake induced by the solar wind at a range of downstream distances; • Measurements of selected lunar crustal magnetic anomalies; • Measurements of the lunar exosphere and surface charging; • Coordinated measurements with NASA’s forthcoming Lunar Atmosphere and Dust Environment Ex- plorer (LADEE) to characterize the near-terminator exosphere. It is desired to get as many measurements as possible in each area so as to maximize spatial resolution, capture variations as the Moon moves in and out of the Earth’s magnetotail, and observe under varying solar wind conditions. By having two ARTEMIS spacecraft in orbit, simultaneous two-point measurements can be achieved at a range of spatial separations, calibration can be conducted, and an increased data volume is obtained. The above science measurement objectives directly drive the design of the lunar science orbit and approach from Lissajous. To achieve the desired lunar wake measurements, the two spacecraft should be in highly eccentric orbits that precess at rates different from each other and from the Sun direction. The mission duration should be long enough so that a full range of measurement altitudes and relative orientations can be achieved. To measure the crustal magnetic anomalies, periapsis altitudes must be less than 50 km and in the vicinity of the anomaly. Each anomaly dictates a minimum orbit inclination required to achieve this. Measurements of the lunar exosphere require a range of altitudes and orientations with respect to the Sun. To coordinate near-terminator exosphere investigations with LADEE, an ARTEMIS spacecraft must be below 200 km within 30 deg of the dawn terminator at least once during the 3-month LADEE science mission scheduled for 100 days between July 2013 and March 2014 (depending on launch date) [8, 9]. Finally, to maximize the quantity of all measurements, the orbits should have long-term stability. SPACECRAFT DYNAMICS Dynamic Model for Trajectory Integration The above discussion on the orbit necessary to achieve the ARTEMIS science goals necessitates a lunar orbit with a large semi-major axis and a high eccentricity. The following accelerations on the spacecraft are used to accurately integrate the spacecraft dynamics in the design process: • Lunar pointmass potential; • Earth pointmass potential; 2 • Lunar harmonic gravity terms (up to 20th degree and order); • Solar pointmass potential; • Solar radiation pressure (SRP, spherical spacecraft model); • Spacecraft thrust. Relativistic accelerations and a higher-order gravity terms for the Earth are found to be small enough to ne- glect in the modeling. Figure 1 shows the relative magnitude of the various natural accelerations on the P2 spacecraft during its transfer from Lissajous through the science orbit (the accelerations on P1 are qualitatively similar). The magnitude of the first three accelerations listed above oscillate over several orders of magnitude as the spacecraft moves between periapsis and apoapsis (on the order of once per day). The Earth gravity magnitude also oscillates twice per lunar month as the spacecraft line of apsides rotates through 360 deg with respect to the Earth-Moon direction. The relative solar gravity oscillates every 9:5 or 17 months (P1 and P2, respectively), primarily driven by variation in the distance between the spacecraft and the Earth. The SRP acceleration oscillates annually, primarily driven by the range between the Earth and Sun. Figure 1. Magnitude of accelerations on the P2 spacecraft (relative to the Moon) from four days before LOI through the end of the PRM sequence. The logarithmic scale for the y-axis on both plots is identical; they are separated for clarity. Qualitative Dynamical Insights Figure 1 shows that the lunar and terrestrial pointmass gravities are the dominant accelerations in the dynamics. The non-Keplerian motion of the ARTEMIS probes while in lunar orbit can be qualitatively understood by considering only these two accelerations in the context of the Hill three-body problem (H3BP)y [10]. Such simplification allows for analytical expressions of the dynamics to inform the design process. When considered in this light, the ARTEMIS dynamics can be understood much the same ways that Jupiter and Saturn moon orbiters have been in the literature [11, 12, 13]. Firstly, the short-period influence of the Earth’s gravity as a function of orbit orientation should be understood. When considering the Moon-centered H3BP dynamics in a frame that rotates so the Earth is always yThe H3BP relies on the assumptions that the second primary is much smaller than the first and that spacecraft

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